The deposition of this type of layer has formed the subject-matter of various studies. The first route consists of depositing it at atmospheric pressure, the precursor of the layer (monomer, prepolymer) being used in the liquid phase. This choice exhibits a number of disadvantages related to the use of solvents: residual traces of solvent may remain in the layer, the solvent has to be reprocessed/recycled and the thickness of the layer is not very easy to control. Finally, it is not always simple to find the solvent capable of correctly wetting the substrate to be coated.
A second route then consists of depositing the layer under vacuum: the precursor is vaporized and then sprayed under reduced pressure over the substrate. The precursor subsequently polymerizes, either spontaneously or under the effect of a specific treatment, in particular a heat treatment, using infrared radiation, for example, or treatment with ultraviolet radiation or by electron bombardment. This technique exhibits numerous advantages in comparison with the preceding one: no longer solvent to be reprocessed, layers comprising less in the way of impurities. It is generally possible to exert better control over the thickness thereof, very particularly in the range from 10 nm to 1 micrometer. In addition, this technique makes it possible to use starting reactants which are sensitive to water or to oxygen or to use combinations of reactants with a very high reactivity and/or which are difficult to mix homogeneously, which are immiscible or only slightly miscible. In addition, the deposition can be included in an operation for the manufacture of multilayer stacks on the same vacuum deposition line, without interrupting the manufacturing operation. Various embodiments of these techniques are disclosed, in particular, in Patents U.S. Pat. No. 5,260,095, EP-340 935, EP-339 844 and EP-733 919. However, this technique is not without disadvantage either: the polymer layers thus deposited tend to exhibit an inadequate mechanical strength and they tend to become detached, to an increasingly more pronounced extent as their thickness enters the submicronic range and/or when they are based on fluoropolymer or silicone polymer.
An aim of the present invention is consequently to overcome this disadvantage, in particular by improving the quality of the polymer layers deposited under vacuum, in particular with regard to mechanical quality.